Ceramic material

ABSTRACT

A porous granular ceramic material has a granule size of from 1 to 5 mm, a bulk density of from 200 to 700 g/l, preferably 350 to 550 g/l, a porosity of from 50 to 80% v/v, preferably from 65 to 80% v/v, and a liquid absorbency of from 20 to 200%, preferably from 40 to 150%, of water based on the weight of the granular material. The material may be made by calcining a mixture of a clay and a particulate combustible material. The material is suitable for use as animal litter or as an industrial absorbent for liquids.

The present invention relates to porous ceramic materials and to methodsfor their production. In particular, the present invention provides aporous ceramic material having absorbent properties which, at least whenin a granular form, make it suitable for such end uses as animal litteror industrial absorbent for liquids.

The present invention provides a porous granular ceramic material havinga granule size predominantly within the range of from 1 to 5 mm, a bulkdensity within the range of from 200 to 700 g/l, a porosity within therange of from 55 to 80% v/v, and a liquid absorbency within the range offrom 20 to 200% of water based on the weight of granular material. Thematerial preferably has a bulk density within the range of from 350 to700 g/l, and a liquid absorbency within the range of from 40 to 150%,more preferably 70% to 150%, of water based on the weight of granularmaterial. More preferably, the material has a bulk density of from 350to 550 g/l, and a porosity of from 65 to 80% v/v.

In one aspect of the invention, in such a porous granular ceramicmaterial the pores are constituted by voids derived from the combustionof a combustible material previously contained therein. The saidcombustible material is advantageously lignite. The ceramic material ispreferably a clay-based material.

The present invention further provides a method for the preparation of aporous ceramic material comprising calcining a mixture of clay and aparticulate combustible material, the temperature and time ofcalcination being such that a substantial amount of the clay mineralsremain unvitrified and at least a part of the combustible material isburned off. Preferably, when the product is to be used as an animallitter, substantially all the combustible material is burnt off.

The temperature of calcination is preferably in the range from 800° C.to 1100° C., more preferably from 1000° C. to 1100° C.

As examples of clays suitable for use in the invention there may bementioned ball clay, especially carbonaceous clay.

When the porous material of the invention is produced from a mixture ofclay and lignite the ratio of clay:lignite is preferably within therange of from 1:9 to 9:1, more preferably from 1:2 to 2:1.

The mixture of clay and a combustible material may be obtained by mixingparticulate or powdered clay and combustible material with sufficientwater to cause the particles to form agglomerates, and drying theagglomerated mixture.

If desired or required, a surfactant or deflocculating agent may beemployed to assist in forming a satisfactory mixture.

The aqueous material is preferably dried to a moisture content of notmore than 5%, wet basis. The mixture is preferably dried at atemperature of not greater than 200° C. Advantageously, the conditionsof the calcination step are such that the weight loss of the mixture isfrom 40 to 65%, preferably 40 to 55%, based on the weight of the driedmixture.

In a preferred process according to the invention the moist mixture isshaped prior to drying and, if necessary, the dried mixture is dividedinto granules. The mixture may, for example, be shaped by extrusion, forexample using a pug mill or brick extruder, and the extrudate dried,broken and screened to recover granules within the desired size range.Any material screened out may be recycled for use so that the processproduces very little waste.

In a particularly advantageous manner of operation the mixture isextruded onto a flat surface so that the granules have at least one flatsurface. This allows more efficient packing of the granules in a bedand, when used as animal litter, the improved packing reduces liquidpenetration through the bed.

Alternatively, the moist mixture may be formed directly into granules.Granulation may occur in the mixer or the mixture may be subjected topelletisation for example on a drum or pan pelletiser.

Preferably, the particle sizes of the clay and of the combustiblematerial in the starting mixture are each not greater than 2 mm.

Some carbonaceous clays are associated geologically with lignite andsuch clays may contain sufficient lignite to be used in their raw statein the method of the invention without the addition of furthercombustible matter. In that case such a clay may be used alone in placeof the combination of clay and combustible material.

Preferably, when the product is to be used as animal litter,substantially all the combustible material is burned off duringcalcination. With many clays, e.g., carbonaceous clays, this enables awhite product to be obtained.

Thus, in accordance with a preferred embodiment of the invention theporous, granular ceramic product is made by forming a moist mixturecomprising clay and lignite, shaping, drying and subdividing the mixtureto form granules, and calcining the granules to burn out the lignite.

The product usually has a substantially neutral pH and is usually strongenough to resist attrition and dust formation in use. The material hasan open-pore structure, the matrix of which is permeable. That is, thematerial contains a network of relatively large pores that areinterconnected by relatively small pores.

The properties of the product depend upon a variety of factors, whichmay be adjusted in order to obtain a product of particular desiredcharacteristics. These factors include, for example: the type of clayemployed, which affects the colour and strength of the product; theparticle sizes of the clay and combustible material, which determine thepore size and the absorption properties; the ratio of clay tocombustible material, which controls the density and hence the porosityand strength; the drying time and temperature, which may affect theparticle shape and also the strength; and the kiln environment (that is,the temperature, firing profile and atmosphere), which can affect thecolour, density, strength and absorption properties. For example, anincreased strength may be obtained in a material according to theinvention by using a higher drying temperature or a higher kilntemperature.

Where an extrusion procedure is employed, the type of extruder willaffect the strength and particle shape of the final product. The type ofcomminuting or crushing device used to break the material after shapingand drying will also affect the particle shape and size distribution ofthe product.

The following examples illustrate the invention:

EXAMPLE 1

50 kg of milled clay (moisture content 28% wet basis), 90 kg milledlignite (moisture content 31.8% wet basis) and 14.7 liters of water areplaced in an Eirich mixer and mixed for 5 minutes. The resulting moistgranular mixture is fed into a pug mill or brick extruder, whichcompresses the mixture and extrudes it in the form of a continuous rodor rods. The rods are passed through a drier at 100° C. until themoisture content is reduced to not more than 5% wet basis. This hardensthe rods so that they may be crushed and sieved to recover granules of asize range of 1-5 mm. The selected granules are finally calcined bypassage through a rotary kiln at 1050° C. until substantially all thelignite is burned off.

The product of this process is a porous granular ceramic material ofirregular shape, with a granule size within the range of from 1 to 5 mm.A bulk product density of 460 g/l is achieved by this process. Theproduct is of substantially neutral pH and is capable of absorbing 77%of its own weight of water or urine. The porosity of the product iswithin the range of from 55 to 80% v/v. The product has high intrinsicstrength and is very resistant to attrition and dust formation, both dryand in use. The properties of the material reduce the amount ofscattering of material by animals when used as litter. Discoloration ofthe product in use is minimal.

(The bulk density of the product is calculated by determining the weightof a given volume (not less than 500 cm³) of material treated inaccordance with B.S.1460 but using manual tapping. The liquid absorptionis determined by covering the material with liquid, mixing, and leavingto stand for 5 minutes, the residual liquid then being removed. Theabsorption in % is calculated from the material weight (W_(m)) andliquid weight absorbed (W_(l)) according to the equation: (W_(l)×100)/W_(m). The porosity is determined using the same method, exceptthat the porosity is calculated from the material volume V_(m) and thevolume of liquid absorbed V_(l) according to the equation: (V_(l)×100)/V_(m). The volume V_(m) is the volume of the material per se, thatis, excluding the volume between particles. The pH of the material isdetermined by dispersing particles of the material of size less than190! in deionised water and after mixing allowing to stand for at least12 hours, the sample then being stirred for one minute prior tomeasuring the pH of the solution.)

EXAMPLE 2

50 kg of carbonaceous clay (moisture content 28% wet basis) and 90 kg oflignite (moisture content 31.8% wet basis) are milled separately to aparticle size of below 2 mm and then placed in an Eirich mixer with 14.7liters of water and mixed for 2 minutes. A mixture containing discretegranules is obtained. The granules are dried directly at 150° C. for 20minutes. Fines and oversize material are recycled to the mixing stage.The remainder is fired at 1070° C. for 20 minutes in an excess ofoxygen.

The product obtained has all the properties of that of Example 1, exceptthat the strength is marginally less. This process avoids the use ofextruder machinery and is therefore cheaper to operate. Furthermore,water or urine absorption of the product may in some cases be greaterthan that of an extruded material made from a mixture of the samecomposition.

EXAMPLE 3

50 kg of milled carbonaceous clay (moisture content 28% wet basis), 90kg of milled lignite (moisture content 31.8% wet basis), and 14.7 litersof water are placed in an Eirich mixer and mixed for 5 minutes. Theresulting moist granular mixture is extruded in the form of a continuoussheet by a brick extruder or a twin roller extruder onto the belt of acontinuous dryer. The sheet is passed through the dryer at 150° C. untilthe moisture content is not more than 5% wet basis. The embrittled sheetis further broken by a roller crusher and after sieving granules of asize range of 1-5 mm are recovered. Fines are recycled to the mixingstage and oversize material is recycled to the crusher. The remainder iscalcined by passing through a rotary kiln at 1050° C. untilsubstantially all the lignite is burned off.

The product has similar properties to that of Example 1 except that thegranules now have at least one flat surface. This allows the formationof an improved packed bed and, when used as animal litter, reducesliquid penetration through the bed.

EXAMPLE 4

50 kg of clay, 30 kg of lignite and 10 liters of water are placed in amixer and mixed for 15 minutes. The resulting moist granular mass is fedinto a brick extruder, which compresses the granules and extrudes themin the form of a continuous rod or rods. The rods are passed through adrier at 100° C. This hardens the rods so that they may be crushed andsieved to recover a particles in the size range 1 to 5 mm. The selectedparticles are finally calcined by passage through a rotary kiln at 1050°C.

The product of this process is a granular ceramic material of irregularshape, porous appearance, and a particle size within the range 1 to 5mm. Bulk product densities of 250 to 650 g/l may be achieved by thisprocess, but are preferably between 350 and 500 g/l. The product is ofneutral pH and may absorb 70 to 200%, preferably 100 to 150%, of its ownweight of water or urine. The product has high intrinsic strength and isvery resistant towards attrition and dust formation, both dry and inuse. The material is therefore not scattered or trodden about by animalswhen used as litter. Discoloration of the product in use is minimal.

EXAMPLE 5

50 kg of carbonaceous clay and 30 kg lignite are milled to below 2 mmand then placed in a mixer with 10 liters of water. After 15 minutesmixing, discrete granules are formed which can be dried directly at 150°C. for 10 minutes. Fines and oversize material are recycled to themixing stage. The remainder is fired at 1070° C. for 10 minutes in anexcess of oxygen.

The product obtained has all the qualities of that of Example 4, exceptthat the strength is marginally reduced. This process avoids the use ofextruder machinery and is therefore cheaper to operate. Furthermore, theliquid absorption capability of the product is greater than that of anextruded material, being between 100 and 300% weight for weight, andmore usually 150 to 250%. The use of carbonaceous clay gives a whitefired product.

EXAMPLE 6

50 kg of carbonaceous clay is dispersed in 20 liters of water with theaid of 100 g sodium silicate as deflocculant. 35 kg lignite isseparately dispersed in 20 liters of water with mixing. The twocomponents are then mixed together for 15 minutes and the resultant massextruded in sheet form onto the belt of a continuous drier. Drying at105° C. for 15 minutes embrittles the sheet, causing it to fractureunder its own weight. The broken pieces are conveyed onto a vibratoryscreen which collects material 1-5 mm in size, rejects fines, andrecycles oversize material. The 1-5 mm fraction is fired at 1100° C. for2 minutes in a toroidal bed kiln.

The product has similar properties to that of Example 4 except that thegranules now have one flat surface. This enables them to form a betterpacked bed and, when in use as animal litter, reduces liquid penetrationthrough the bed.

What we claim is:
 1. A method for the preparation of porous granularceramic material having a granule size predominantly within the range offrom 1 to 5 mm, a bulk density within the range of from 200 to 700 g/l,a porosity within the range of from 55 to 80% v/v, and a liquidabsorbency within the range of from 20 to 200% of water based on theweight of granular material, the pores in said porous material beingconstituted by voids derived from the combustion of lignite previouslycontained therein, said method comprising calcining granules comprisinga mixture of particulate clay and particulate lignite in a weight ratioof 1:2 to 2:1, the temperature and time of calcination being such that asubstantial amount of the clay remains unvitrified and the lignite issubstantially completely burnt off and the weight loss of the mixture isbetween 40 and 65% based on the weight of the dried mixture.
 2. A methodaccording to claim 1, wherein the calcination temperature is from 800°C. to 1100° C.
 3. A method according to claim 1, wherein the mixture ofclay and lignite is obtained by mixing clay, the lignite and water toobtain a moist agglomerable mixture and drying agglomerates formed fromthe mixture.
 4. A method as claimed in claim 3, wherein the mixture isdried to a moisture content of not more than 5%.
 5. A method accordingto claim 3, wherein the aqueous mixture is dried at a temperature notexceeding 200° C.
 6. A method according to claim 1, wherein the mixtureis shaped prior to drying and the dried mixture is divided intogranules.
 7. A method according to claim 6, in which the mixture isshaped by extrusion, and the extrudate is dried, broken and screened torecover granules within a preselected size range.
 8. A method accordingto claim 7, in which the mixture is extruded onto a flat surface.
 9. Amethod according to claim 1, in which clay, lignite and water are mixedtogether, the amount of water being such that the mixture so obtained isin the form of discrete granules, which are dried and screened prior tocalcination.
 10. A method according to claim 1, in which the particlesizes of clay and the lignite in the starting mixture are each notgreater than 2 mm.
 11. A method according to claim 1, in which granuleswithin the size range of 1 to 5 mm are calcined.
 12. A method for theproduction of animal litter comprising the steps of:a) forming granuleshaving a granule size predominately within the range of from 1 to 5 mmand comprising an intimate mixture of particulate clay and particulatelignite in a weight ratio of from 1:2 to 2:1, said clay and ligniteparticles having a particle size of not greater than 2 mm and saidgranules having a moisture content of not more than 5%; b) calciningsaid granules at a temperature of from 80° C. to 1100° C. for a periodof time such that i) the weight loss from the granules is from 40 to 65%by weight, ii) the lignite is substantially burnt out of the granulesand iii) the clay minerals remain substantially unvitrified; and c)recovering porous granular ceramic material having a granule sizepredominantly within the range of from 1 to 5 mm, a bulk density withinthe range of from 200 to 700 g/l, a porosity within the range of from 55to 80% v/v, and a liquid absorbency within the range of from 20 to 200%of water based on the weight of granular material, the pores in saidporous material being constituted by voids derived from the combustionof the lignite.